This invention relates to a current-input type interface circuit of a mouse, especially for one interface circuit in which current parameters instead of voltage parameters are utilized to constitute the input parameters for a mouse.
The presently available mice all have a ball to frictionally drive two rollers to respectively correspond to the shift of the mouse in X and Y directions. Each roller coaxially carries a photogrid (optic interrupter) which co-rotates with the roller, thus the X and Y displacements can be photoelectrically converted into corresponding electrical signals. Two photo-coupling sensors each cooperates with one of the corresponding roller to sense the X displacement or Y displacement.
Referring to FIG. 4, either of the photo-coupling sensors comprising a light emitting diode (LED) 51 and a photo-transistor 52, which are respectively arranged at the opposite sides of the photogrid device, generates pulsative signal similar to a train of pulses. The frequency of the pulse-like signal represents the displacement of X-axis (Y-axis), thus reflecting the displacement and the direction of the mouse. Referring to FIG. 3, a mouse-input sensing circuit 53 receives the pulse-like signal from the photo-transistor 52 and transmits the signal to a control/processing circuit 54 for converting the pulse-like signal into a standard digital signal. An output circuit 56 is provided to promote the signal level of the digital signal from the control/processing circuit 54 and to give an output signal receivable/readable by a computer.
The mouse-input sensing circuit 53 used at the present time comprises a resistor R and an inverter 531, as shown in FIG. 5. The resistor R is connected between the emitter electrode of the photo-transistor 52 and a grounding point. The input of the inverter 531 is also connected to the emitter electrode of the photo-transistor 52. When the photo-transistor 52 receives the light emitted from the diode 51, an electrical signal is produced, so a potential is obtained across the resistor R. The signal is further inverted by means of the inverter 531 and outputted at the output terminal 0/P for driving subsequent circuits. Therefore, the mouse-input sensing circuit 53 cooperates with the photo-transistor 52 to generate required voltage to drive the subsequent circuits. However, the mouse-input sensing circuit 53 is integrated in a mouse control chip, the manufacturing process of which affects the stability of the input voltage and the response speed as described below.
Due to the manufacturing process of the integrated circuit, the transition point of the inverter 531 has a relatively large range of errors; also the receiving characteristic of each photo-transistor 52 has its individual difference; the conducting current of the photo-transistor is relatively small (possibly lower than 5 micro-ampere), thus causing the insufficient tolerance of the transition of the inverter 531. For example, if the transition point of the inverter 531 is 1.4 volts, the resistor R should be 280 K-ohm during the minimum current (5 micro-ampere). The relatively high resistance (280 K-ohm) of the resistor R requires a relatively large area in the integrated circuit, contrary to the trend of miniaturization for integrated circuit. Moreover, due to the offset of the manufacturing process, the practical resistance might have an error up to 20%, i.e., the real resistance of the resistor R might vary in the range between 233 K-ohm and 336 K-ohm. If the resistance is 233 K-ohm, the output voltage is only 1.16 volts, which is not enough to enable the inverter 531 to transit its status. One could solve this problem by increasing the resistance of the resistor R. However, the increased resistance will lead to reduction of the V.sub.CE value of the photo-transistor to a too small value, thus causing the transistor 52 to enter its saturation status and decreasing its response speed. For example, when the photo-transistor 52 receives a total bombardment of the emitted light, the current therethrough might be as high as 40 to 50 micro-ampere. If the resistor is 280 K-ohm (notice that the 280 K-ohm falls in the above mentioned range between 233 K-ohm and 336 K-ohm), the potential across the resistor R is up to 11.2 volts, which is greater than the source voltage 5-volt. Therefore, the voltage will actually not undergo the above situation, but remain in a certain equilibrium point, which renders the V.sub.CE relatively small, thus causing the photo-transistor 52 to enter its saturation status. At this time, if the photo-grid device rotates in a too high speed, then the photo-transistor will not be able to catch up to timely respond with a switching over (from ON to OFF, or vise versa), thus causing the malfunction (wrong interpretation) of the mouse. Therefore, the resistance of the resistor R of the conventional voltage-input sensing circuit must neither be too great nor too small, and must be restricted in a narrow range. However, such an accuracy in resistance is practically very difficult for circuit integration.